Great advancements have been made in semiconductor technology in the last few years largely due to the discovery of new materials and the discovery of new methods of making better materials. These advances have led to new device applications for semiconducting materials and such applications often require different fabrication techniques. Typically, these techniques are aimed toward obtaining smaller size, more precise location of various geometrical features in the device, more accurate shapes for various geometrical features in the structure, greater adherence of metallic substances to the semiconductor surfaces, etc.
An important aspect of semiconductor fabrication technology is etching procedures and solutions. Etching solutions should be stable, have predictable rates and produce uniform etching rates over wide areas of a semiconductor wafer. A particularly important problem in the fabrication of many III-V semiconductor devices is the uniform etching of layers (generally epitaxial layers) of aluminum containing III-V semiconductor compounds (e.g. aluminum gallium arsenide, aluminum indium arsenide, etc.). Such compounds often exhibit problems both as to the stability of the etching solution and uniformity of the etch rate across the semiconductor wafer. In addition, in the fabrication of many devices, it is highly advantageous to selectively etch one semiconductor (e.g. an aluminum containing semiconductor compound) without etching (or etching only to a small extent) another semiconductor compound.
A particular case in point is the development of a variety of heterojunction devices involving different III-V semiconducting compounds. A typical example is heterojunction devices featuring aluminum gallium arsenide and gallium arsenide. The inclusion of the aluminum is done for a variety of reasons including altering the band gap, providing lattice matching to another III-V semiconductor compound, improve etch unformity and changing optical properties of the semiconductor.
In fabricating such devices, it is highly desirable to be able to selectively etch one III-V semiconductor compound without etching the other semiconductor compound. For example, in fabricating various gallium arsenide heterojunction devices, it is often highly desirable to etch aluminum gallium arsenide selectively without etching the gallium arsenide. In many device fabrication procedures, it is desirable that such selective etching be effective even for very low aluminum contents. Often, this is desirable because crystal morphology is easily degraded with high aluminum content material. Also, various electronic properties change as aluminum content increases beyond 20 or 30 mole percent.
A variety of references describe etching solutions and processes for III-V semiconductor compounds. Particularly noteworthy is U.S. Pat. No. 4,049,488 issued on Sept. 20, 1977 to R. P. Tigburg. This reference describes selective etching of aluminum gallium arsenide in the presence of gallium arsenide using various redox systems at specific pH valves. Also described is selective etching of gallium arsenide in the presence of aluminum gallium arsenide.
Other references also describe etching of gallium aluminum arsenide including "GaAs-GaAlAs Heterojunction Transistor for High Frequency Operation" by W. P. Dumke et al, Solid State Electronics, 1972, Volume 15, pp. 1339-1343; "Selective Etch Characteristics of HF for Al.sub.x Ga.sub.1-x As/GaAs", by X. S. Wu et al, Electronic Letters, 21, 558 (1985), "Extreme Selectivity in the Lift-Off of Epitaxial GaAs Films", by Eli Yablonovitch et al, Applied Physics Letters, Volume 51, No. 26 (1987); and "The Etching of Ga.sub.0.7 Al.sub.0.3 As Using KI-I.sub.2 " by A. C. Wismayer et al, Materials Letters, Volume 6, page 284-286 (1988).
Aqueous dichromate ion solutions have been used in etching III-V semiconductor compounds in a number of references including "Chemical Etching of (100) GaAs in the (NH.sub.4).sub.2 Cr.sub.2 O.sub.7 --H.sub.2 SO.sub.4 --NH.sub.4 Cl--H.sub.2 O Systems", by I. Barycka et al, Journal of Material Science 21 (1986) 2153-2158; "A New Etchant System, K.sub.2 Cr.sub.2 O.sub.7 --H.sub.2 SO.sub.4 --HCl, for GaAs and InP" by S. Adachi et al, Journal of Materials Science 16, 2449-2456 (1981); "Defect Selective Etching of GaAs in CrO.sub.3 --HCl Solutions" by J. van de Ven et al, Chemtronics, 1986, Vol. 1, March, pp. 19-26; "The Mechanism of GaAs Etching in CrO.sub.3 --HF Solutions" by J. van de Ven et al, Journal of the Electrochemical Society, 132, 3020 (1985) and "Chemical Etching on GaAs" by S. Adachi et al, Journal of the Electrochemical Society: Solid- State Science and Technology 131, No. 1, pp. 126-130 (1984)